Pneumonia virus of mice (PVM) infects mice, rats, hamsters and guinea pigs. Because of the broad spectrum of rodent hosts, PVM has the potential to influence many experimental results, notably those studies involving the cell dynamics of pulmonary parenchyma. The objective of this project is to study the natural history and transmission of PVM in euthymic and athymic mice. A supply of stock PVM virus (ATCC VR-25, Strain No. 15) has been produced in BHK-21 cells and in nude mice (10% lung suspension) for future studies. Studies were undertaken to determine means and duration of virus transmission from experimentally infected CD-1, C3H/Hen and nu/nu mice to susceptible cage mates (added at specified intervals post infection) and to other mice housed in adjoining cages with or without filter bonnets. Mice were infected by the nasal route with 0.04 ml of the appropriate virus suspension. Preliminary results indicate that mortality rates in the CD-1 mouse differ from those of the C3H/Hen and nude mouse when infected with the ATCC strain of PVM virus. Preliminary results also indicate that the ATCC strain of PVM is not readily transmitted to cage mates even after three passages in the nude mouse. None of the mice housed in adjacent cages, with or without filter bonnets, seroconverted to PVM. These preliminary results indicate that PVM is not easily transmitted among mice. A stock supply of a wild or street strain of PVM isolated from naturally infected nude mice will be produced for comparative studies with the ATCC strain.